A device of this type and use thereof are known in principle from DE 10 2005 045 569 A1.
Devices for generating short-wavelength radiation based on a discharge plasma which is generated in a pulsed manner are used, for example, to implement lithography methods, particularly when it is a question of high repetition frequency of the discharge processes. At the same time, fluctuations in the amount of radiation emitted are to be limited to less than 1% to ensure a consistent quality of the emitted radiation.
The electric voltage, typically several kilovolts, required for generation of a discharge plasma is supplied via a (resonant) charging circuit with a pulsed capacitor or capacitor bank. In so doing, a charging switch of the charging circuit is closed at a first switching time and the pulsed capacitor is charged. A virtually sinusoidal current flow through the inductor develops during this phase. At a second switching time, the charging switch is opened and a freewheeling switch is closed so that the energy stored in the inductor is decreased through a further rise in voltage at the pulsed capacitor. At a third point in time which is defined by the current zero crossing of the inductor, the plasma is ignited, i.e., a flow of current is initiated between two electrodes between which the plasma is to be generated.
In this way, the energy stored by the charging circuit for generation of the discharge plasma is provided as a discharge voltage at two electrodes which are separated by a gap.
When an electrically conductive channel is generated or provided between the electrodes at a firing time, the available energy flows along the channel, thereby exciting emission of a plasma of an emitter material. At the firing time, the discharge voltage will have reached a maximum. An electrically conductive channel can be produced through the evaporation of an emitter material, e.g., tin, by means of a high-energy radiation, e.g., a laser. In so doing, the emitter material can be present, for example, on the electrodes or can be introduced, for example, by injection of droplets, therebetween.
In devices for generating discharge plasmas, it is difficult in actual practice to coordinate the first and second switching times and the firing time in such a way that high repetition frequencies are accompanied by minimal deviations in the amount of radiation emitted by the discharge plasma. The reason for this consists in fluctuating parameter values, for example, voltage differences, different component values and variable plasma efficiency, i.e., the extent of conversion of electrical energy into radiation energy. In this respect, parameter values which change only slightly over a number of pulses may differ from those which do not change only slightly from pulse to pulse. As a rule, the parameter values which change slightly show a certain tendency in the direction of their changes (long-term drift), while the parameter values which change from pulse to pulse usually vary stochastically.
The device suggested in DE 10 2005 045 569 A1 has a resonant charging circuit with at least one charging switch for charging a charging capacitor of the resonant charging circuit and at least one discharging switch for discharging the resonant charging circuit. Further, a control is provided which calculates the switching times of the charging switch in real time depending on input values and one or more reference quantities. In order to reduce the required computing effort for coordinating the switching times, DE 10 2005 045 569 A1 suggests relying for calculation upon an approximation algorithm or on lookup tables which were calculated and set up in a non-time-critical method segment. In a time-critical method segment, actual measurement values, e.g., of the charging voltage, can be taken into consideration in real time and the switching times can be determined. The charging switch is triggered based on the calculated first and second switching times.
By means of the device according to DE 10 2005 045 569 A1, the second switching times are calculated in such a way that the desired discharge voltage is supplied at a precisely fixed firing time.
For calculating the second switching times and the firing time, it is known to apply semianalytic methods to a simplified model of a resonant charging circuit. First, a relationship is determined between a desired charging voltage and the second switching time. This relationship is typically nonlinear and can be described by a polynomial. The firing time can be determined using another, analytic relationship. This analytic relationship in which the second switching time is entered further contains simplifying assumptions about the behavior of the resonant charging circuit.
A disadvantage in a procedure of this kind consists in that the effects of various elements of the resonant charging circuit are not sufficiently acquired, if at all, due to the simplifications. Thus the effect of an existing degaussing circuit is not taken into account. Further, the DC voltage source is assumed to be ideal.
Due to these simplifications, significant errors can occur in calculating the second switching time and firing time which must be corrected subsequently by introducing optimization factors.
In the procedure mentioned above, it is also very difficult or even impossible to determine the relationship between the desired discharge voltage and the second switching time after changes in the topology of the resonant charging circuit. Every change in topology requires a new determination of the relationship, and complex topologies cannot be described at all by this procedure.